Current limiting fuse



Oct. 19, 1965 F. CAMERON CURRENT LIMITING FUSE 3 Sheets-Sheet 1 FiledFeb. 25, 1962 INVENTOR Frank L. Cameron BY M 4. WCLLQL ATTORNEY Oct. 19,1965 Filed Feb. 23, 1962 TIM E IN SECONDS L. m u a m 5 lllllll ix: '0:In in F. L. CAMERON 3,213,242

CURRENT LIMITING FUSE 3 Sheets-Sheet 2 Fig. 4.

IllllFl lll llll I000 2600 4600 liooo Oct. 19, 1965 F. L. CAMERONCURRENT LIMITING FUSE 3 Sheets-Sheet 3 Filed Feb. 23, 1962 my v mm b mmhm United States Patent C 3,213,242 CURRENT LIMITING FUSE Frank L.Cameron, North Huntingdon Township, Westmoreland County, Pa, assignor toWestinghouse Eleotric Corporation, East Pittsburgh, Pa., a corporationof Pennsylvania Filed Feb. 23, 1962, Ser. No. 175,103 4 Claims. (Cl.200-420) This invention relates, generally, to electric circuitinterrupting devices and, more particularly, to interrupters of a typeknown as current limiting fuses which are connected to limit the currentupon interruption of heavy overloads and short circuits.

For effective operation as a current limiting interrupter a fuse mustmelt on the steeply rising slope of the current wave before the firstcurrent peak of the alternating current wave is reached. The rate ofrise of this current wave and the arc voltage generated and maintainedby the fuse are the essential ingredients of its successful operation.

In accordance with standards established by Electrical Manufacturers,specific melting characteristics are required of a power fuse to qualifyit for an E-rating and assure its electrical interchangeability. Thefuse must also have the ability to interrupt the considerably lowermelting currents associated with the E-rating characteristic.

An object of this invention is to provide a power fuse which is simplein construction, which meets the requirements of standards establishedby Electrical Manufacturers, which is suitable for relatively highvoltage application and which has a high full load current rating.

Current limiting fuses are frequently utilized on lowcost transformerapplications where the total space provided for the fuse is small, andwhere the cost of the fuse must be low. A further requirement, somewhatat variance with the preceding, is that the fuse must have a highinterrupting rating.

Another object of the invention is to provide a current limiting fusewhich is compact, low in cost, and has a high interrupting rating.

Still another object of the invention is to provide a fuse havingspecific melting characteristics and a good low current clearingability.

Other objects of the invention will be explained fully hereinafter orwill be apparent to those skilled in the art.

In accordance with one embodiment of the invention, a desirable currentor E-rating characteristic and the ability to interrupt the considerablylower melting currents associated with the E-rating characteristic areobtained by sandwiching a layer of CaCO (calcium carbonate) in a fusebetween layers of a silica sand of controlled grain size. The fuseelements are notched silver ribbons prebent in a zig-zag manner to makethem fatigueproof. In another embodiment of the invention, the fuseelement is a stepped or graduated silver wire constructed to generate aspecific arc voltage and obtain a high interrupting rating. A layer ofCaCO is utilized between layers of sand in this fuse to obtain specificmelting characteristics and a good low current clearing ability.

For a better understanding of the nature and objects of the invention,reference may be had to the following 3,213,242 Patented Oct. 19, 1965detailed description, taken in conjunction with the ac companyingdrawings, in which:

FIGURE 1 is a view, partly in elevation and partly in section, of a fuseconstructed in accordance with one embodiment of the invention;

FIG. 2 is a view, in plan, of an end terminal and fuse element space-rutilized in the fuse of FIG. 1;

FIG. 3 is an edgewise view of a portion of one of the fuse elementsutilized in the fuse;

FIG. 4 is a group of time-current characteristic curves for the fuse ofFIG. 1;

FIG. 5 is a view, partly in section and partly in elevation, of a fuseconstructed in accordance with another embodiment of the invention, and

FIG. 6 is a view, in elevation, of a stepped wire utilized as the fuseelement in the fuse of FIG. 5, the difference in wire diameters beingexaggerated.

Referring to the drawing, the fuse shown in FIG. 1 is generally similarto the one described and claimed in my copending application Serial No.85,872, filed January 30, 1961, which issued May 26, 1964, as US. Patent3,134,874, and which is assigned to the same assignee as the presentapplication. It comprises a tubular fuse holder 4 having end caps 5 and6 secured over opposite ends of the tube 4 and a current limiting andare extinguishing unit 7 disposed inside of the tube 4 which may becomposed of a glass melamine material. The end caps 5 and 6 arepreferably composed of copper and they are silver plated over theirentire outer surface.

The unit 7 comprises a tubular rod 3 composed of insulating material,such as ceramic, with end terminal members 10, 11 on opposite ends ofthe rod 8. The ends of the rod 8 have reduced portions 9, 9 each adaptedto be received within a centralized mating aperture in one of theterminal members 10, 11. The end terminals may be composed of anyconducting material, preferably copper. The rod 8 may be attached to theterminals 10, 11 in any suitable manner, as by an adhesive.

As shown in FIG. 2, each terminal member 10, 11 is a disc of sufiicientdiameter to extend substantially across the inside of the tubular holder4. Each disc is provided with a pair of spaced conductor studs 12, 12,preferably of 4 inch copper, extending perpendicularly to the disc formating relationship with suitable apertures 13, 13 in the end caps 5, 6.The studs may be connected to the end caps at the apertures 13, 13 inany suitable manner, but preferably the studs are electrically connectedto the end caps or ferrules by means of a mechanical spin-press fitwhich is then soldered to forestall any possibility of corrosion at themechanical joint.

Each terminal disc 10, 11 is provided at its outer periphery with twelveradially disposed, equally spaced narrow slots 14. Each slot 14 in oneterminal disc is aligned with .a slot 14 in the opposite terminal disc,each pair of aligned slots being provided with a fusible element 15extending therethrough. The ends of the fusible elements 15 are fittedin the aligned slots and bent over at the ends which may be soldered tothe copper end termi nal if desired.

Each fusible element 15 is ribbon-like in shape and is preferablycomposed of silver. The ribbon is segmented by V-notching on both edgesperiodically along its length to provide a series of restricted areaswhich fuse to provide a series of arcs, the summation of which providesa high are voltage as is known in the art. By way of example, the silverribbon element may be inch wide by .005 inch thick and provided withV-notches having an included angle of about 36 at 4 inch intervals alongits length. It will be understood that the length of the fusibleelements depends upon the voltage of the system in which the fuse is tobe utilized.

Referring now to FIG. 3, there is shown the zig-zag appearance of thefusible element 15 resulting from preindexing or prebending of theelement periodically along its length each successive bend being equaland opposite to the adjacent bends relative to a central axis along thefusible element, Investigation with non-indexed ribbon type fusibleelements have shown that expansion and contraction of the fusibleelement effected pronounced flexing at one or two points along theelement, ultimately causing the element to fracture or break underfatigue failure.

Preindexing of the fusible element, as shown in FIG. 3, effects uniformdistribution of the flexing among the various indexed points with theresult that variation in stress intensity at any one point is keptwithin the endurance range of the metal, and the element does not break.Preferably, the fusible element should be bent as sharply as possible atthe points of indexing without damaging the surface of the wire. It isapparent that the minimum number of indexed points is dependent upon thelength of the fusible element and the amount of expansion andcontraction anticipated for the particular material. As an example offatigue proofing of a fusible element through prebending or preindexingas shown, a silver strap of the type hereinbefore described, having alength of ten inches, was indexed at about A inch intervals to anincluded angle of about 120 resulting in a fuse having fatigue proofelements.

The construction of the fuse as hereinbefore described wherein theterminals and 11 may be attached to the support rod 8 and the fuseelements 15 as a self-supporting subassembly permits the testing of theinterrupting unit 7 alone before insertion into the tubular holder 4.This is an important advance in manufacturing as it permits the visualinspection and a culling out of any subassemblies which may bedefective, before final assembly within the holder 4.

In order to provide a visual indication of fuse operation, an operationindicator 17 is disposed at one end of the tube 4. The indicator 17comprises a cylindrical housing 18 having a shoulder 19 at one endabutting the terminal disc 11 and having a tubular portion extendingbeyond the shoulder 19 within the previously described centralizedaperture in the disc 11. The opposite end of the housing 18 is open andextends outwardly of disc 11 into abutting relationship with the insideedge of an aperture 20 in the end cap 6, the aperture 20 having adiameter slightly smaller than the inside diameter of the housing 18 forreasons which will be immediately apparent. Slidably mounted within thehousing 18 is a cylindrical plastic indicator 21 having a diameterslightly less than the diameter of aperture 20 and having a flangeportion 22 at the inner end thereof having a diameter substantially thesame as that of the inside of housing 18 and greater than the diameterof aperture 20 in the end cap 6. The length of the cylindrical indicator21 is such that it may be fully enclosed within the housing 18, thusindicating the absence of fuse operation. A compression spring 23 ismounted in the housing 18 between the shoulder 19 and the flange 22 ofthe indicator 21 to bias the indicator outwardly of the housing 18whereupon the flange 22 engages the inside edge of aperture 20, thusindicating the operation of the fuse by the extension of indicator 21outwardly of the housing 18 into view of an inspector.

Normally, the indicator 21 is held within the housing 18 out of view ofthe inspector by means of a fusible conductor 24 fixed to an upper stud12 and extending downwardly through the central aperture of tubularsupport rod 8, through the centralized aperture of terminal disc 11,through a tapered neck opening 25 in housing 18, an eye 26 on the innerend of indicator 21, back through the opening 25 and between the outsideof the shoulder 19 and the underside of the disc 11. Thus, the conductor24 is pressed into conducting relationship with the disc 11 when thefuse is assembled. A tapered plug 27 is tightly fitted in the taperedopening 25, thereby serving as an anchor for the end of the conductor 24to hold the indicator 21 in the retracted position against the biasingaction of the spring 23. The housing 18 and the plug 27 are composed ofinsulating material.

In operation, when the main fusible elements 15 fuse, an increasedcurrent is passed through conductor 24 causing it to fuse, therebyreleasing the indicator 21 which ex tends outwardly of housing 18 underthe biasing force of spring 23. Inasmuch as the lower looped portion ofconductor 24 extending between the plug 27 and the eye 26 is in airrather than in an arc extinguishing means, as is the remaining portionof the conductor, the described looped portion fuses first rather thanthe remaining portion to effect release of the indicator 2i.

Heretofore, the practice has been to fill fuses of the present type withsand. In order to obtain the desired current or E-rating characteristicin the present fuse and the ability to interrupt the lower meltingcurrents associated with the E-rating characteristic, a layer of CaCO(calcium carbonate) is sandwiched in the fuse between layers of a silicasand of controlled grain size. The minimum current required to causemelting of a given fuse ele: ment can be controlled by changing thethickness and/ or position of the layer of CaCO The most effectivecontrol is achieved by varying the thickness of the layer. As anexample, a fuse built with three current limiting elements each composedof a notched silver strap .005 of an inch thick by inch wide is a 65Eampere fuse when constructed with a three inch layer of CaCO This samefuse has an -E ampere rating when the CaCO layer is reduced to twoinches in thickness, an example of E rating curves is shown in FIG. 4.

This change in melting characteristic is achieved because of themodified heat flow which results from use of the CaCO layer. The veryfine CaCO powder is so compacted during the fuse assembly that the heatgenerated by the fuse element in the section blanketed by the CaCO layeris less effectively dissipated. This in effect lowers the currentrequired to cause melting of the fusible element.

Equally important is the manner in which the CaCO aids in low currentinterrupt-ions. At a temperature slightly below the melting point ofsilver the CaCO decomposes and gives off a gas which aids in areinterruption. A fuse built with a layer of CaCO has the ability toeffectively interrupt currents well below those which can be interruptedby a sand filler alone. Since the CaCO decomposes instead of fusing, nofulgurite is formed. Furthermore, CaCO has a peculiar advantage overother materials, such as gypsum and boric acid, in that it will begin todecompose only at a temperature significantly higher than that at whicheither one of the aforementioned materials decomposes. Thus, the gas isgiven off by the CaCO at a time when it is most effective ininterrupting the are.

In building the fuse, the interrupter and the indicator subassembliesare combined and the lower ferrule or end cap press-fitted into place.The glass melamine tube is next inserted into the ferrule and the tubeis partly filled with sand 16 to a desired height, such as the distanceA in FIG. 4. Next, CaCO indicated by 16', is put into the tube toprovide a layer having a desired thickness such as the distance B whichencompasses the longitudinal center of the fusible element. Then,sufficient sand 16 is put into the tube to fill it, as indicated by thedistance A. An opening 28 is provided in each terminal disc to aid ingetting the filler material into the tube 4.

Assurance of an adequate and compact filling may be attained byutilizing a vibrator-filler which completely fills the fuse, except forthe indicator. The top ferrule is next fitted over the tube andpress-fitted to the upper end studs. Final soldering of the ferrule-studconnection is followed by a baking operation which cures the cement usedbetween the ferrules and the glass melamine tube. The fuse is then readyfor final inspection and checking.

As explained hereinbefore, a current limiting fuse constructed in theforegoing manner has a high full load current rating and it has theability to interrupt the relatively low melting currents associated withthe current rating charateristic.

The fuse illustrated in FIG. 5 is also of the current limiting type. Itcomprises a tube or holder 31, two end caps or ferrules 32, a core 33, afusible element 34 wound on the core 33 and filler material comprisingtwo layers of sand 35 with a layer 36 of CaCO sandwiched between the twolayers of sand. The tube 31 is preferably composed of borosilicate glasshaving heat resisting properties. The ferrules 32 are preferablycomposed of copper and they may be cemented onto the ends of the tube.The core 33 is of a relatively low cost and it may be made by coatingfilament glass tape with a layer of sodium silicate (water glass). Thefusible element 34 is a composite silver wire having stepped orgraduated diameters. Thus, the fuse contains no organic material.

In order to generate a specific arc voltage during interruption, thecomposite fusible element 34 comprises three sections E, F and G, eachsection being of different length and having a different diameter. Asshown in FIG. 6, the three sections may be welded together. By way ofexample, the section B may have a length of 8% inches, sect-ion F alength of 4 inches and section G a length of 5% inches, therebyproviding a composite Wire 18 inches in length. The diameter of sectionE may be .0045 of an inch, that of section P .0056 and that of section G.005 of an inch. Thus, each section has a different fusingcharacteristic.

As each diameter of the wire melts in turn, the molten wire continues toact as a conductor. There is a considerable increase in resistance asthe element changes from a solid to a liquid. There is consequently alarge increase in the energy developed at the moment the fuse meltscausing the entire section of the element to vaporize. The spaceformerly occupied by the wire is now filled with a metal vapor of hightemperature which is momentarily an insulator. With the sudden insertionof this insulator in the circuit, a high voltage is developed in theinductance of the circuit, and the voltage causes an arc to be formedacross the vapor column. The fuse arc voltage now increases rapidlybecause of the refractory :material in the fuse which both confines anddeionizes the arc.

As shown in FIG. 5, a pin 37 is attached to each end of the core 33 bymeans of a rivet 38. The fuse wire 34 is wound on the core and each endof the wire is soldered to one of the pins 37. A ferrule 32 is placedover one end of the core with a pin 37 extending through an opening inthe end of the ferrule. One end of the tube 31 is pressed into theferrule and cemented in place. The tube is then filled with two layersof sand with a layer of CaCO sandwiched between the layers of sand. Theother ferrule is cemented onto the other end of the tube with the pin 37extending through the ferrule. The pins are soldered to the ferrules,thereby sealing the openings through which the pins extend. Aftersoldering, the ends of the pins may be cut off next to the solder.

As explained hereinbefore, the thickness of and the location of the bandof CaCO provide specific melting characteristics and a good low currentclearing ability. The present fuse is relatively small in size and lowin cost. By way of example, a fuse suitable for 5 kv. service and ratedat 2 amperes has an overall length of 7 inches. The layer of CaCO at thecenter, designated by B is 2 inches thick. Each layer of sand,designated by A,

d is approximately 2 inches thick. Thus, the CaCO encompasses the middlesection of the fusible conductor. This fuse is particularly suitable foruse on transformer applications where the total space for the fuse issmall and it is desirable that the cost be low, but the fuse must have ahigh interrupting rating.

From the foregoing description, it is apparent that the utilization of alayer of CaCO sandwiched between two layers of silica sand in a fuse hasdefinite advantages as set forth in the foregoing. The utilization offusible elements of the type described in conjunction with the CaCOmakes it possible to build fuses having specific operatingcharacteristics.

Since numerous changes may be made in the abovedescribed constructionand different embodiments of the invention may be made without departingfrom the spirit and scope thereof, it is intended that all mattercontained in the foregoing description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

I claim as my invention:

1. A fuse comprising, a tubular fuse holder, spaced terminal meanssupported by said holder, a ribbon-like fusible conductor havingperiodically spaced restricted portions along its length connectedbetween said terminal means, said conductor being prebent in a zig-zagmanner across the plane of the conductor along substantially its entirelength to distribute flexing of the conductor among the zig-zaggedportions of the conductor, and arc-quenching means comprising a layer ofcalcium carbonate powder disposed in the central portion of said fuseholder and finmly packed between two layers of silica sand around theconductor in the holder.

2. In a fuse, in combination, a tubular fuse holder, an insulating corecentrally disposed inside the holder, a ferrule on each end of theholder, a terminal member at each end of the core, each terminal memberbeing electrically connected to one of the ferrules, a three-sectionstepped fusible conductor wound around said core and connected betweenthe terminal members, the diameter of the middle section of theconductor being greater than the diameter of either one of the endsections, and arcquenching means comprising a layer of compacted calciumcarbonate powder disposed in the central portion of said fuse holder andsandwiched between two layers of silica sand surrounding the conductorin the holder.

3. In a fuse, in combination, a tubular fuse holder, an insulating corecentrally disposed inside the holder, a ferrule on each end of theholder, a terminal member at each end of the core, each terminal memberbeing electrically connected to one of the ferrules, a three-sectionstepped fusible conductor wound around said core and connected betweenthe terminal members, the diameter of the middle section of theconductor being greater than the diameter of either one of the endsections, a layer of compacted calcium carbonate powder disposed in thecentral portion of said fuse holder and substantialy encompassing themiddle section of the conductor, and a layer of silica sandsubstantially encompassing each one of the end sections of the conductorin said holder.

4. A fuse comprising, a tubular fuse holder, spaced terminal meanssupported by said holder, a ribbon-like fusible conductor havingperiodically spaced restricted portions along its length connectedbetween said terminal means, said conductor being prebent in a zig-zagmanner across the plane of the conductor along substantially its entirelength to distribute flexing of the conductor among the zig-zaggedportions of the conductor, and arc-quenching means comprising a layer ofcalcium carbonate powder disposed in the central portion of said fuseholder and firmly packed between two layers of silica sand around theconductor in the holder, said tubular fuse holder being composed ofborosilicate glass.

(References on following page) References Cited by the Examiner UNITEDSTATES PATENTS Cole 200-120 Lemmon 200-135 Hill 200-120 Lohausen 200-120X Lohausen 200-120 Triplett 200-135 Hart 200-135 Lohausen 200-131 Suitset a1 200-131 X Dryer 200-131 Kozacka 200-120 Kozacka 200-120 Burt eta1.

Fister.

Brown 200-120 10 BERNARD A. GILHEANY, Primary Examiner.

1. A FUSE COMPRISING, A TUBULAR FUSE HOLDER, SPACED TERMINAL MEANSSUPPORTED BY SAID HOLDER, A RIBBON-LIKE FUSIBLE CONDUCTOR HAVINGPERIODICALLY SPACED RESTRICTED PORTIONS ALONG ITS LENGTH CONNECTEDBETWEEN SAID TERMINAL MEANS, SAID CONDUCTOR BEING PREBENT IN A ZIG-ZAGMANNER ACROSS THE PLANE OF THE CONDUCTOR ALONG SUBSTANTIALLY ITS ENTIRELENGTH TO DISTRIBUTE FLEXING OF THE CONDUCTOR